This invention relates to composite materials and more particularly to tough, impact-resistant fiber-reinforced composites. Still more particularly, this invention relates to toughened composites comprising continuous fiber embedded in unique epoxy matrix resin formulations toughened with novel particulate modifiers, and to methods for producing such composites.
Fiber-reinforced composites are high strength, high modulus materials which are finding wide acceptance for use in sporting goods and in producing consumer items such as appliances. Composites are also finding increased acceptability for use as structural components in automotive applications, as components of buildings and in aircraft. When used in structural applications the composites are typically formed of continuous fiber filaments or woven cloth embedded in a thermosetting or thermoplastic matrix. Such composites may exhibit considerable strength and stiffness, and the potential for obtaining significant weight savings makes them highly attractive for use as a metal replacement.
The composites industry has long been involved in finding ways to further improve the mechanical properties of composite materials used in structural applications. Considerable effort has been expended over the past two decades directed toward the development of composites with improved fracture toughness. Inasmuch as most of the commonly employed matrix resins, as well as many of the reinforcing fibers, are generally brittle, much of that effort has gone into a search for components having better toughness characteristics. As a consequence, the search for toughened matrix resins has become the subject of numerous recent patents and publications, and numerous formulations have been made available to the composite industry through these efforts.
The methods used for toughening engineering resins have been adapted for the toughening of the matrix resins commonly used in composite structures, as shown for example by Diamant and Moulton in "Development of Resin for Damage Tolerant Composites--A Systematic Approach", 29th National SAMPLE Symposium, Apr. 3-5, 1984. The forming of alloys and blends by adding a more ductile thermoplastic such as a polysulfone to an epoxy resin formulation has also been shown to improve the ductility of the epoxy resin and provide enhanced toughness, according to British patent 1,306,231, published Feb. 7, 1973. More recently, combinations of an epoxy resin with terminally functional thermoplastics were shown to exhibit enhanced toughness. See U.S. Pat. No. 4,498,948. Still more recently, curable combinations of epoxy resins and thermoplastics with reactive terminal functionality were also said to improve the toughness of specifically formulated matrix resins, provided that the neat resin after curing exhibits a specific phase-separated morphology having a cross-linked glassy phase dispersed within a glassy continuous phase. See U.S. Pat. No. 4,656,208. Further improvements are said to be achieved by including a reactive rubbers component which is said to be contained within the cross-linked dispersed glassy phase. See U.S. Pat. No. 4,680,076. Still more recently, the use of an infusible particle made from a rubber dispersed within the phase-separated cross-linked epoxy resin matrix has been suggested for toughening composites based on such matrix resins. See U.S. Pat. No. 4,783,506.
Although the addition of rubber, thermoplastics and the like generally improves the ductility and impact resistance of neat resins, the effect on the resulting composites is not necessarily beneficial. In many instances the increase in composite toughness may be only marginal, and a reduction in high temperature properties and in resistance to environmental extremes such as exposure to water at elevated temperatures is frequently seen.
An alternative approach to producing toughened composites has been the development of layered composite structures having layers formed of fibers imbedded in a matrix resin alternated with layers formed of a thermoplastic resin, described in Japanese patent application 49-132669, published May 21, 1976. More recently, in U.S. Pat. No. 4,604,319, there were disclosed layered fiber-resin composites having a plurality of fiber-reinforced matrix resin layers inter-leafed with thermoplastic layers adhesively bonded to the reinforced matrix resin layers. Inter-leaf structures are ordinarily produced by impregnating continuous fiber to form prepreg, then laying up the composite by alternating prepeg with sheets of thermoplastic film. The laid-up structure is then subjected to heat and pressure, curing the matrix resin and bonding the layers. The patent also discloses inter-leaf layers which comprise a thermoplastic filled with a reinforcing material such as chopped fibers, solid particles, whiskers and the like.
Although inter-leafed composite structures with improved toughness have been disclosed, there has been some sacrifice in other physical properties, including a reduction in glass transition temperatures together with an increase in creep at high temperatures. Further difficulties with such composites may include a loss in stiffness for many such compositions, adhesive failure that may occur between layers formed of dissimilar resins and property deterioration during use due to poor solvent resistance. In addition, prepregs based on thermoplastic resin generally are lacking in tack, which complicates their fabrication into composites and increases the degree of skill needed to fabricate complex structures. This may in turn result in increased scrap losses and a need for more complex quality control procedures, increasing manufacturing costs in order to achieve an acceptable level of reliability.
Recently, the use of an infusible particle made from a rubber dispersed within a phase-separated cross-linked epoxy resin matrix has been suggested for toughening composites based on such matrix resins. See U.S. Pat. No. 4,783,506. Dispersing rigid particulate modifiers in the matrix resin has also been disclosed in the art for toughening composite materials, and has been described for examples in published European Patent Applications 0 274,899 and 0 351,025 as well as in U.S. Pat. No. 4,863,787, the teachings of these latter three publications being hereby incorporated by reference.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partly cured matrix resin. In order to be useful in commercial fabrication operations, prepreg needs to have a long out-time, defined as the period of time the prepreg can remain at room temperature and still be useful for making composites. For use in layups with complex contours the prepreg also must be pliable, and remain pliable in storage. Preferably the prepreg surface will also have and retain good tack. Pliability in prepreg is conferred by the matrix, which should remain soft and deformable to avoid cracking during fabrication.
The matrix resins most widely used for such prepreg systems are epoxy-based formulations, and many comprise an epoxy resin and aromatic amine hardener. The aromatic diamine hardener preferred for a wide variety of commercial applications has been 4,4'-diaminodiphenyl sulfone (DDS). DDS has a low level of reactivity with epoxy resins at room temperature, and prepreg made using DDS-based epoxy resin formulations generally has the desired long out-times. However, most epoxy matrix resin formulations based on DDS require further modification to overcome the low toughness that is characteristic of composites made from these resin formulations.
The isomeric form of DDS, 3-3'-diaminodiphenyl sulfone or 3,3'-DDS, is known in the art to be an effective hardener for epoxy resins. The reactivity of 3,3'-DDS is generally greater than DDS, and epoxy formulations based on this diamine generally have very short shelf life due to the greater reactivity. Although composites made from epoxy formulations based on 3,3'-DDS are known to exhibit improved toughness, the shorter shelf life makes the manufacture of useful prepreg from such formulations a much more difficult task. Alternative diamines having lower reactivities, as well as a variety of cure inhibitors for use in slowing the cure rate of these highly reactive systems, have also become available to formulators of matrix resins, and some of these have found acceptance in the art. In order to produce fully-cured composites and attain the maximum possible toughness and resistance to environmental attack, many slow-cure systems require extended curing cycles and post-curing operations, and often require temperatures well above the 350.degree. F. curing temperature ordinarily preferred by the composite fabricating art. Such formulations are not preferred by fabricators, and have not been well-accepted.
Several methods for improving the damage tolerance characteristics of composites thus are known in the art. However, many of these prior art compositions including those toughened by including particulate modifiers may be poor in other important properties such as fabricability. Moreover, even when toughened to provide good damage tolerance, many laminate structures fabricated from these prior art compositions exhibit failure due to inadequate interlaminar strength characteristics. Improved composite materials having better resistance to impact, better compressive strength after impact and a high level of interlaminar strength as measured by G.sub.IC and methods for their preparation are thus needed. In the fabrication of such improved composites, matrix resins with extended shelf life and out-times capable of being fully cured in conventional fabricating operations using 350.degree. F. curing cycles are needed to permit better handling and more practical storage, so that prepreg made from such resins would have the improved storage-stability needed for the production of complex layups. Such improved composite material formulations could find rapid acceptance by resin formulators and composite manufacturers, displacing the more complex composite materials currently available for these purposes as well as the expensive and difficult manufacturing processes used in their manufacture.